Formation of substances by the developing blastocysts and the role of the latter in implantation is presently controversial. We have demonstrated, by the tritium exchange assay, the aromatization of tritiated testosterone by microsomes of hamster blastocysts, endometrium myometrium and liver (approximately 70-80 hrs. post-ovulation), with formation of (3H) metabolites as indicated by HPLC. The (3H) androstenedione was noted, and (3H) estradiol from incubation of blastocysts and myometrium. Blastocysts were 24-fold more active than the endometrium and myometrium in synthesizing 3H2O. Recrystallization confirmed identities of (3H)-2-methoxyestradiol, (3H)-androstenedione and (3H)-estradiol. Development of aromatization ability in earlier stages of development (Day 2 or 3) in ova from pregnant, pseudopregnant and cyclic hamsters will be investigated. The effect of aromatization inhibitory substances upon implantation will be investigated. The effect of systemic, intrauterine lumen injection or culture of blastocysts in or with aromatization inhibitors on subsequent development and implantation will be investigated. Extension of this to the mouse or rat is planned. The effect of histamine inhibitors and estrogen-antagonists on hamster implantation will be continued.